William R. Saunders

Active suppression of acoustic radiation from impulsively excited structures

The objective is to use active control to suppress the acoustic energy that is radiated to the far field from a structure that has been excited by a short‐duration pulse. The problem is constrained by the assumption that the far‐field pressure cannot be directly measured. Therefore, a method is developed for estimating the total radiated energy from measurements on the structure. Using this estimate as a cost function, a feedback controller is designed using linear quadratic regulator theory to minimize the cost. Computer simulations of a clamped–clamped beam show that there is appreciable difference in the total radiated energy between a system with a controller designed to suppress vibrations of the structure and a system with a controller that takes into account the coupling of these vibrations to the surrounding fluid. The results of this work provide a framework for a general, model‐based method for actively suppressing transient structural acoustic radiation that can also be applied to steady, narro...

Vibration Control through Passive Constrained Layer Damping and Active Control

To add damping to systems, viscoelastic materials (VEM) are added to structures. In order to enhance the damping effects of the VEM, a constraining layer is attached, creating a passive constrained layer damping (PCLD) treatment. When this constraining layer is an active element, the treatment is called active constrained layer damping (ACLD). Recently, the investigation of ACLD treatments has shown it to be an effective method of vibration suppression. In this paper, the treatment of a beam with a separate active and PCLD element will be investigated. Two new hybrid variations will be introduced. A Ritz-Galerkin approach is used to obtain discretized equations of motion. The damping is modeled using the Golla-Hughes-McTavish (GHM) method and the system is analyzed in the time domain. By optimizing on the performance and control effort for both the active and passive case, it will be shown that hybrid treatment is capable of lower control effort with more inherent damping, and is therefore a better approach to suppressing vibration than ACLD.

An Examination of the Relationship Between Chemiluminescent Light Emissions and Heat Release Rate Under Non-Adiabatic Conditions

Abstract : Combustion instability research has matured over the last decade and with it the need for more detailed diagnostics has increased. One main gap in the diagnostics is the ability to obtain a reliable quantitative measure of unsteady heat-release rate. In an effort to move in this direction using chemiluminescence as the measured quantity, this paper examines the formation of chemiluminescence light in premixed flames under non-adiabatic conditions. The main chemiluminescence emitters considered in the study are OH and CH. Experimental results for two types of burners are reported, a laminar Bunsen burner with co-flow and a ceramic honeycomb flat flame burner. The study shows that although the chemiluminescence observed in the two burners behaves very differently with respect to changes in experimental variables, the variation can be fully understood. OH chemiluminescence is found to be a good indicator of heat-release in both burners, whereas CH chemiluminescence is shown to be insensitive to some changes in heat- release rate. Based on the experimental results, the notion that chemiluminescence yield behaves linearly with flow rate cannot be universally supported. The non-linear variation observed is shown to correspond to an equally non- linear variation of heat-release with flow-rate. The results of the study thus have important ramifications for the interpretation of chemiluminescence measurements in dynamic combustion environments.

Stability and operating constraints of adaptive LMS-based feedback control

The filtered-X LMS algorithm has enjoyed widespread usage in both adaptive feedforward and feedback controller architectures. For feedforward controller designs the filtered-X LMS algorithm has been shown to exhibit unstable divergence for plant estimation errors in excess of +/-90^o. Typical implementations of this algorithm in adaptive feedback controllers such as filtered-U and filtered-E have previously been assumed to conform to these same identification constraints. Here we present two instability mechanisms that can arise in filtered-E control that violate the 90^o error assumption: feedback loop instabilities and LMS algorithm divergence. Analysis of the adaptive feedback system indicates that the conventionally interpreted plant estimation error can be arbitrarily small yet induce algorithm divergence; while other cases may have very large estimation errors and feedback loops cause controller instability. These analytical observations are supported by simulations. The implications of the actual plant estimation error, calculated here for the filtered-E controller, are extended to practical constraints placed on applications including filtered-U, on-line system identification, and self-excited system control.

Experiments in piezostructure modal analysis for MIMO feedback control

An approximate method for modal analysis of a piezostructure testbed is used to generate a dynamic model for closed-loop, multiple-input-multiple-output (MIMO) feedback controller design. An innovative pole-residue system model for structures instrumented with piezoelectric sensor and actuators is developed which is compatible with existing modal curve-fitting algorithms. The authors examine the use of the new pole-residue model in the absence of truly collocated response information. It is shown that nearly-collocated measurements may be used to estimate a structures modal parameters; high-precision signal conditioning electronics required for exact drive-point response measurements are thereby avoided. A simply-supported plate is used to demonstrate the approximate piezostructure modal test approach. The test model is then used to design up to a four input, sixteen channel output MIMO feedback control experiment. Closed-loop results are presented which show that more than 10 dB of suppression is achieved near structural resonances within the control bandwidth (10-250 Hz).

Modeling active constrained-layer damping using Golla-Hughes-McTavish approach

Viscoelastic material (VEM) adds damping to structures. In order to enhance the damping effects of the viscoelastic material, a constraining layer is attached. If this constraining layer is a piezoelectric patch, the system is said to have active constrained layer damping (ACLD). In this paper, the damping effects due to viscoelastic material which has an active constraining layer is modeled using the Golla-Hughes-McTavish (GHM) damping method. The piezoelectric patch and structure are modeled using a Galerkin approach in order to account for the effect of the constraining layer on the beam.

Structural Acoustic Control of a Shape Memory Alloy Composite Beam

Active control of sound radiation from a clamped, baffled, composite beam with embedded Shape Memory Alloy (SMA) fibers was demonstrated using two dif ferent control strategies. The unique behavior of the SMA reinforced composites was uti lized to allow minimization of radiated sound for harmonic beam vibration and placement of peak radiation response at specified frequencies within a controllable range.

The use of tunable diode laser absorption spectroscopy for the measurement of flame dynamics

Tunable diode laser absorption spectroscopy was used to measure temperature fluctuations in acoustically forced laminar and turbulent flames. The absorption of two high-temperature water lines, at 7444.37 cm−1 (v1+v3 bands) and 7185.59 cm−1 (2v1, v1+v3 bands), yielded an instantaneous temperature measurement of the product stream. The instantaneous temperature of the gases was used as an indicator of the energy transferred to the product stream from the combustion process. The frequency response of product gas temperature to velocity perturbations was compared to the frequency response of OH* chemiluminescence, an indicator of the chemical heat release rate. Past measurements of flame dynamics used chemiluminescence as the sole indicator of heat release rate, in effect assuming that the energy input rate from the flame into the acoustic field is dynamically equivalent to the chemical reaction rate. Through the use of TDLAS, the unsteady enthalpy of the gases was measured, which includes the effects of thermal diffusion and heat transfer. The measurements show that the frequency response function of gas temperature differs significantly from the chemiluminescence frequency response.

Dynamic Analysis of Swirl Stabilized Turbulent Gaseous Flames

With the advent of lean premixed gas turbine combustors, research in the area of thermo-acoustic instabilities and active combustion control came into the limelight. To be able to predict and control these instabilities, it is required that both the acoustics of the system, and a frequency-resolved response of the combustion process to velocity perturbations be understood. Experimental techniques developed by the Virginia Active Combustion Control Group at Virginia Tech, to obtain an open loop flame transfer function were applied to both fully and partially premixed swirl stabilized turbulent gaseous flames using commercial grade methane as fuel. A frequency-resolved fluctuating velocity was applied at the inlet of the combustor within the frequency range of 20–400 Hz, and the OH* chemiluminescence was used as a measure of the fluctuating heat release rate within the flame. Experiments were conducted at atmospheric pressure for two swirl numbers of 0.79 and 1.19, and three equivalence ratios of 0.55, 0.60 and 0.65. The flow rates studied resulted in Reynolds numbers of 14,866 and 19,821. The results show that for the linear range, the magnitude of the FRF is primarily dependent on the premixing quality and the mean energy content of the mixture, while the phase of the FRF is quite sensitive to Φ′ oscillations and to the variations in the species concentration across the cross-section of the flow.Copyright

Perspectives on linear compensator designs for active combustion control

Some of the earliest research in active combustion control (ACC) showed that the use of simple phase-shifter circuits feeding back acoustic pressure to voice-coil actuators was sufficient to achieve some reduction of the acoustic pressure caused by thermoacoustic instabilities in atmospheric combustors. Since that time, many researchers have continued to use phase-shifter controllers to suppress pressure fluctuations for a variety of combustion testbeds. In addition, other researchers have proposed the use of somewhat more’ sophisticated linear controllers and demonstrated their capabilities in reducing peak pressures. All of the ACC results motivate a series of interesting questions from a linear systems theory perspective: Why does a simple phase inverter controller generally ‘work’ for this problem? What is the effective Gequency response function for the phase-shifter compensator and how does it impact the controlled response? Can critical performance characteristics of the controlled system (magnitude of suppression and occurrence of controller-induced instabilities) be predicted a priori? Are there common features between the simple phase shift controller and other linear controllers that have been successful? Are there ‘untried’ linear controller designs that are motivated by these analyses and existing results for suppression of instabilities in combustors? This paper focuses on a subset of these perspectives, relying on the use of relevant linear control theory concepts to provide some answers and illuminate the need for more extensive nonlinear analyses (the subject of future publications) for predicting certain information. Specifically, this paper Copyright 63 1999 by t+ American Institute of Aeronautics and Astronautics provides a detailed discussion of the phase-shifter control method that has been so popular for active combustion control. Analytical and experimeutal considerations demonstrate how to predict frequencies and approximate amplitudes of controller-induced instabilities (also referred to as ‘spillover’ or ‘secondary peaks’) when using acoustic control and a phase-shifter compensator. This investigation also illustrates the intluence of the phase-shifter controller on the degree of controllability achieved for acoustic control of thermoacoustic insrabilities in a simple tube combustor.